Magnesium is a highly reactive and thermodynamically unstable element. Molten magnesium is readily and violently oxidised in ambient air, burning with a flame temperature of approximately 2820° C. Three approaches have been used to inhibit the severe oxidation process. Salt cover fluxes may be sprinkled over the molten metal; oxygen may be excluded from contacting the molten metal by blanketing the molten metal with an inert gas such as helium, nitrogen or argon; or a protective cover gas composition may be used to blanket the molten metal. Protective cover gas compositions typically comprise air and/or carbon dioxide and a small amount of an inhibiting agent which reacts/interacts with the molten metal to form a film/layer on the molten metal surface which protects it from oxidation. To this day, the mechanism by which inhibiting agents protect molten reactive metals is not well understood.
U.S. Pat. No. 1,972,317 relates to methods for inhibiting the oxidation of readily oxidisable metals, including magnesium and its alloys. The patent notes that at the time of its filing in 1932, numerous solutions had been proposed to the oxidation problem including displacing the atmosphere in contact with the metal with a gas such as nitrogen, carbon dioxide or sulphur dioxide. U.S. Pat. No. 1,972,317 teaches inhibition of oxidation by maintaining in the atmosphere in contact with molten metal an inhibiting gas containing fluorine, either in elemental or combined form. Reference is made to many fluorine containing compounds with the solids ammonium borofluoride, ammonium silicofluoride, ammonium bi-fluoride and ammonium fluophosphate or the gases evolved therefrom upon heating being said to be preferred. Notwithstanding the issue of U.S. Pat. No. 1,972,317 in 1934, it was not until about the mid-1970's that a fluorine containing compound found commercial acceptance as an inhibiting agent in a cover gas.
Prior to about the mid-1970's, sulphur dioxide (SO2) was widely used as an inhibiting agent in a magnesium cover gas composition but was replaced by sulphurhexafluoride (SF6) which has become the industry standard. Typically, SF6 based cover gas compositions contain 0.2–1% by volume SF6 and a carrier gas such as air, carbon dioxide, argon or nitrogen. SF6 has the advantages that it is a colourless, odourless, non-toxic gas which can be used for protecting molten magnesium/magnesium alloy and in the production of bright and shiny ingots with relatively low dross formation. However, SF, suffers from several disadvantages. Its sulphur based decomposition products at high temperature are very toxic. It is expensive, has limited sources of supply, and is one of the worst known greenhouse gases having a Global Warming Potential (GWP) at a time horizon of 100 years of 23,900 relative to 1 for carbon dioxide.
It is also noted that once magnesium has ignited, the resulting fire cannot be extinguished even with high concentrations of SF6. SO2 is even worse in this respect as it can accelerate a magnesium fire. The only known cover gas for extinguishing a magnesium fire is boron trifluoride (BF2) which is very expensive and very toxic.
Alternative cover gas compositions are desirable.